C++20
C++20 is the informal name for the revision of the ISO/IEC standard for the C++ programming language expected to follow C++17.[1] The standard became technically finalized[2] by WG21 at the meeting in Prague in February 2020.[3] The standard was expected to be officially published after the end of the DIS ballot in May 2020, but as of July 2020, it is still underway.[4]
C++20 adds more new major features than C++14 or C++17.[5] Below is a partial list of changes that have been accepted into or have been discussed for inclusion into C++20.[6]
New Features
C++20 introduced many new features. The following lists may be incomplete.
Language
- concepts,[7] with terse syntax[8]
- modules[9]
- designated initializers[10] (based on the C99 feature, and common G++ extension)
[=, this]
as a lambda capture[11]- template parameter lists on lambdas[12]
- three-way comparison using the "spaceship operator",
operator <=>
- initialization of an additional variable within a range-based
for
statement[13] - lambdas in unevaluated contexts[14][15]
- default constructible and assignable stateless lambdas[14][16]
- allow pack expansions in lambda init-capture[14][17]
- string literals as template parameters[14][18]
- removing the need for
typename
in certain circumstances[19] - new standard attributes
[[no_unique_address]]
,[20][[likely]]
and[[unlikely]]
[21] - conditional
explicit
, allowing the explicit modifier to be contingent on a boolean expression[22] - expanded
constexpr
: virtual functions,[23] union,[24] try and catch,[25] dynamic_cast and typeid,[26] std::pointer_traits[27] - immediate functions using the new
consteval
keyword[28] - signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[29]
- a revised memory model[30]
- various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[31][32]
- coroutines[33]
using
on scoped enums[34]constinit
keyword[35]
Library
- ranges (The One Ranges Proposal)[36]
std::make_shared
andstd::allocate_shared
for arrays[37]- atomic smart pointers (such as
std::atomic<shared_ptr<T>>
andstd::atomic<weak_ptr<T>>
)[38] std::to_address
to convert a pointer to a raw pointer[39]- calendar and time-zone additions to
<chrono>
[40] std::span
, providing a view to a contiguous array (analogous tostd::string_view
butspan
can mutate the referenced sequence)[41]<version>
header[42]std::bit_cast<>
for type casting of object representations, with less verbosity thanmemcpy()
and more ability to exploit compiler internals[43]- feature test macros[44]
- various constexpr library bits[45]
- smart pointer creation with default initialization[46]
std::map::contains
method[47]- bit operations, such as leading/trailing zero/one count[48], and log2 operations[49][50][51]
Deprecation
- Use of comma operator in subscript expressions has been deprecated[52]
- (most of)
volatile
has been deprecated[53]
New (and changed) keywords
Many new keywords added (and the new "spaceship operator", operator <=>
), such as concept
, constinit
,[35] consteval
, co_await
, co_return
, co_yield
, requires
(plus changed meaning for export
), and char8_t
.[54] And explicit
can take an expression since C++20.[55] Most of the uses of the volatile
keyword have been deprecated.[53]
In addition to keywords, there are identifiers with special meaning, including new import
and module
.
New attributes in C++20:
[[likely]]
,
[[unlikely]]
,
and [[no_unique_address]]
[56]
History of changes
Changes applied to the C++20 working draft in July 2017 (Toronto) include:[57]
- concepts
- designated initializers
[=, this]
as a lambda capture- template parameter lists on lambdas
std::make_shared
andstd::allocate_shared
for arrays
Changes applied to the C++20 working draft in the fall meeting in November 2017 (Albuquerque) include:[58][59]
- three-way comparison using the "spaceship operator",
operator <=>
- initialization of an additional variable within a range-based
for
statement - lambdas in unevaluated contexts
- default constructible and assignable stateless lambdas
- allow pack expansions in lambda init-capture
- string literals as template parameters
- atomic smart pointers (such as
std::atomic<shared_ptr<T>>
andstd::atomic<weak_ptr<T>>
) std::to_address
to convert a pointer to a raw pointer
Changes applied to the C++20 working draft in March 2018 (Jacksonville) include:[60]
- removing the need for
typename
in certain circumstances[61] - new standard attributes
[[no_unique_address]]
,[62][[likely]]
and[[unlikely]]
[63] - calendar and time-zone additions to
<chrono>
[64] std::span
, providing a view to a contiguous array (analogous tostd::string_view
butspan
can mutate the referenced sequence)[65]<version>
header[66]
Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[67]
- contracts deferred to a later standard[68]
- feature test macros[69]
- bit-casting of object representations, with less verbosity than
memcpy()
and more ability to exploit compiler internals[70] - conditional
explicit
, allowing the explicit modifier to be contingent on a boolean expression[71] - constexpr virtual functions[72]
Changes applied to the C++20 working draft in the fall meeting in November 2018 (San Diego) include:
- ranges (The One Ranges Proposal)
- concept terse syntax
- constexpr union, try and catch, dynamic_cast, typeid and std::pointer_traits.
- various constexpr library bits[73]
- immediate functions using the new
consteval
keyword[74] - signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[75]
refinements of the contracts facility (access control in contract conditions)[76] (see list of features deferred to a later standard)- a revised memory model[77]
- Smart pointer creation with default initialization[78]
Changes applied to the C++20 working draft in the winter meeting in February 2019 (Kona) include:[79][80]
- coroutines[81] – already experimentally supported in Clang 5[82]
- modules[83] – experimentally supported in Clang 5[84] and Visual Studio 2015 Update 1[85] as well as GCC[86]
- various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[87][88]
Changes applied to the C++20 working draft in the summer meeting in July 2019 (Cologne) include:[89][90][91]
- contracts have been removed (see list of features deferred to a later standard)[92]
- use of comma operator in subscript expressions has been deprecated[93]
- constexpr additions (trivial default initialization,[94] unevaluated inline-assembly[95])
- using scoped enums[96]
- various changes to the spaceship operator[97][98]
- DR: minor changes to modules[99]
constinit
keyword- changes to concepts (removal of
-> Type
return-type-requirements[100]) - (most of)
volatile
has been deprecated[53] - DR:
[[nodiscard]]
effects on constructors[101] - The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)[102]
- text formatting[103][104] (chrono integration,[105] corner case fixes[106])
- bit operations[107]
constexpr INVOKE
[108]- math constants[109]
- consistency additions to atomics (
std::atomic_ref<T>
,[110]std::atomic<std::shared_ptr<T>>
[111]) - add the
<=>
operator to the standard library[112] - header units for the standard library[113]
- synchronization facilities[114] (merged from: Efficient atomic waiting and semaphores,[115] latches and barriers,[116] Improving atomic_flag,[117] Don't Make C++ Unimplementable On Small CPUs[118])
std::source_location
[119]- constexpr containers (
std::string
,[120]std::vector
[121]) std::stop_token
and joining thread (std::jthread
)[122]
Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:[123][124][125]
- Class Types in Non-Type Template Parameters (NTTP): The restriction of no user-defined
operator==
allowed has been removed as the meaning of template argument equality has been divorced fromoperator==
.[126] This allows also for array members in class-type NTTP. - Floating-point types,[127] pointers and references and unions and union-like classes (class types containing anonymous unions) are now allowed as NTTP.
- Function identity now also includes trailing requires-clauses (P1971)
- Constrained non-template functions have been removed
<compare>
is now available in freestanding implementations[128]std::span
s typedef was changed fromindex_type
tosize_type
to be consistent with the rest of the standard library[129]- Concept traits have been renamed to follow the renaming of the concepts as a result from the Cologne meeting
- Several fixes and additions to ranges (P1456R1: Move-only views,[130] P1391R4: Range constructor for
std::string_view
(constructor from iterator-pair of characters),[131] P1394R4: Range constructor forstd::span<ref>
,[132] P1870R1: forwarding-range<T> is too subtle[133]) - Initialization for
std::atomic<T>
has been changed to make it work with default and list initialization,[134]std::latch
andstd::barrier
can now report the maximum number of threads that the implementation supports through the new member functionmax()
std::weak_equality
andstd::strong_equality
have been removed as they are not used anymore- Algorithms in
<numeric>
have been madeconstexpr
- Missing feature-test macros for new or changed features of C++20 have been added[135]
Features published as Technical Specifications
Features deferred to a later standard
- Contracts – a new study group (SG21) has been formed to work on a new proposal[140]
- Reflection[141][142]
- Metaclasses[143]
- Executors[144]
- Networking extensions,[145][146] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
- Properties[147]
- Extended futures[148]
See also
References
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- ^ Dusíková, Hana (2019-11-06). "N4817: 2020 Prague Meeting Invitation and Information" (PDF). Retrieved 2020-02-13.
{{cite web}}
: CS1 maint: url-status (link) - ^ "Current Status : Standard C++". Retrieved 2020-07-16.
{{cite web}}
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- ^ Howard E. Hinnant; Tomasz Kamiński. "Extending <chrono> to Calendars and Time Zones".
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- ^ Alan Talbot. "<version>".
- ^ Herb Sutter. "Trip report: Summer ISO C++ standards meeting (Rapperswil)".
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- ^ "P1032R1 - Misc constexpr bits".
- ^ "P1073R2 - Immediate functions".
- ^ "P1236R0: Alternative Wording for P0907R4 Signed Integers are Two's Complement".
- ^ "P1289R0 - Access control in contract conditions" (PDF).
- ^ "P0668R4: Revising the C++ memory model".
- ^ "Smart pointer creation with default initialization".
- ^ "r/cpp - 2019-02 Kona ISO C++ Committee Trip Report (C++20 design is complete; Modules in C++20; Coroutines in C++20; Reflection TS v1 published; work begins on a C++ Ecosystem Technical Report)". reddit. Retrieved 2019-02-24.
- ^ "Trip report: Winter ISO C++ standards meeting (Kona)". Sutter’s Mill. 2019-02-23. Retrieved 2019-02-24.
- ^ "N4649: Working Draft, Technical Specification on C++ Extensions for Coroutines" (PDF).
- ^ "Clang 5 Release Notes on coroutines".
- ^ "N4720: Working Draft, Extensions to C++ for Modules" (PDF).
- ^ "Clang 5 documentation on Modules".
- ^ "C++ Modules in VS 2015 Update 1".
- ^ "cxx-modules - GCC Wiki". gcc.gnu.org. Retrieved 2019-02-24.
- ^ "P1091R1: Extending structured bindings to be more like variable declarations". www.open-std.org. Retrieved 2019-02-24.
- ^ "P1091R2: Extending structured bindings to be more like variable declarations". www.open-std.org. Retrieved 2019-02-24.
- ^ "r/cpp - 2019-07 Cologne ISO C++ Committee Trip Report — 🚀 The C++20 Eagle has Landed 🚀 (C++20 Committee Draft shipped; Contracts Moved From C++20 to a Study Group; `std::format` in C++20; C++20 Synchronization Library)". reddit. Retrieved 2019-09-15.
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- ^ "P1161R2: Deprecate uses of the comma operator in subscripting expressions". www.open-std.org. Retrieved 2019-07-20.
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- ^ "P1668R1: Enabling Constexpr Intrinsics By Permitting Unevaluated inline-asm in Constexpr Functions". www.open-std.org. Retrieved 2019-07-20.
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- ^ "P1630R1: Spaceship needs a tune-up". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
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- ^ "P1855R=: Make <compare> freestanding". www.open-std.org. 2019-10-05. Retrieved 2019-11-09.
- ^ "P1872R0: span should have size_type not index_type" (PDF). www.open-std.org. 2019-09-16. Retrieved 2019-11-09.
- ^ "P1456: Move-only views" (PDF). www.open-std.org. 2019-01-25. Retrieved 2019-11-09.
- ^ "P1391: Range constructor for std::string_view" (PDF). www.open-std.org. Retrieved 2019-11-09.
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External links
- JTC1/SC22/WG21 – the ISO/IEC C++ Standard Working Group (a.k.a. the C++ Standards Committee)
- Ranges (range-v3) github repository, by Eric Niebler